9 Answers
9

First off, the ISO standard is not intended to produce a good cup of tea. It is designed to produce a consistent one for taste testing, so that no tea manufacturer can claim that his tea wasn't made "properly".
It's title is "Tea -- Preparation of liquor for use in sensory tests"

As for the actual tea making, yes, leaving the bag in longer will make a stronger cup of tea. The concentration of caffeine (along with flavour molecules and everything else) will slowly trend towards an equal concentration in the leaf and in the water. The longer you leave the tea bag/leaves in the water, the closer to equilibrium you will get.

There are other factors that affect this, such as the temperature of the water, cut of the leaves, bag versus loose leaf and so on, but the trend is always towards equilibrium as time progresses.

I'm not sure where the upper limit of this lies, but I think that once the cup is cold there's no point in it anyway. Thus, leaving the bag in for an hour is a bit much. I usually steep my tea for 3-6 minutes, depending on how strong I want it to be.

The way the Japanese make iced tea is to steep the leaves in cold water, in the fridge, for a day or more. This works for ordinary tea, Earl Grey etc. as well as for green tea. So steeping after the water goes cold definitely has an effect.
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slimMar 9 '11 at 16:11

@slim: The steeping has an effect, naturally. I meant that there was no point in drinking the tea by the time it's cold. Making iced tea is a separate subject entirely.
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CarmiMar 10 '11 at 13:53

I recently observed the effect of time of steeping on caffeine content in tea. We used High Pressure Liquid Chromatography to determine the levels of caffeine in Green Tea in samples that were steeped for 1, 2, 4, 10, 15, 30, 60, and 120 mins. The concentration of caffeine over time did not show any significant trends. Our results suggest that caffeine content does not depend on the steeping time and instead the caffeine dissolves in the hot water within a minutes of starting to steep the tea. Hope this helps.

Did you publish these result on a paper or just as a personal experiment? Either way, I'd love to see the plot if you have it?
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JeffDrorFeb 21 '14 at 13:51

1

It would be nice to see this data or know more about this. There are plenty of other experiments which have looked at the first 30 seconds to 15 minutes of brewing tea, but few that have gone beyond that. Also, it would be odd to find that there are "no significant trends": every other experiment has shown that tea brewed for longer times tends to contain more caffeine. It's just that once you get to 10-15 minutes, over 90% of the caffeine will be extracted, so it can't go up much more. If it actually went DOWN after that, it would be really interesting.
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AthanasiusApr 10 '14 at 16:58

In case you're looking for a scientific approach, this paper has some plots showing the amount of caffeine extracted as a function of temperature and time. Look for the "CA" label in figures 2-5. They steep the bags for 30 seconds at a time and record the relative & cumulative amounts of caffeine (and other ingredients) extracted from the tea.

There have been a number of scientific studies addressing the question of how much caffeine is extracted depending on brewing time. Perhaps the most cited is from 1996, but a 2008 study (carried out to debunk the myth that tea can be decaffeinated with a 30-second steep) also gives some useful data with a variety of teas.

To address your question specifically, approximately 70-80% of caffeine is removed, on average, in a 6-minute steep with boiling or near-boiling water. It will vary depending on variety of tea (green, black, etc.) and form (whole black tea leaves release caffeine the slowest; black tea bags containing tea fannings the fastest). Regarding the 30-second "decaf method," only about 10% of caffeine is released in that short time, so it's hardly effective. You'd need to steep for at least 5 minutes or so to remove a significant portion of the caffeine.

Given the number above for a 6-minute steep, the maximum amount you could expect to extract from a longer steep for hours or days would be 20-30% of the original caffeine content of the leaves. So you could potentially increase the amount of caffeine in the final brew to maybe 1.25 times of the 6-minute cup or a little more, depending on variety.

However, it should be noted that more than 90% of caffeine will be released by 15 minutes, so steeping for hours or days is not very productive. If, for some reason, you wish to extract the most caffeine from the leaves possible, I would recommend multiple short steeps (5 minutes or less) instead, perhaps with a higher concentration of leaves. Using fresh water periodically will allow faster extraction of caffeine, and you'll also avoid the inevitable bitterness that generally comes from a single long brewing.

(I should note all of the above regards typical brewing with relatively hot water. Brewing tea with room temperature water or with cold water will significantly increase the time it takes for caffeine to be extracted. In that case, brewing for hours may be necessary to allow large portions of the caffeine to dissolve.)

the 2008 study says nothing of the speed at which the caffeine is extracted, since tea was steeped for 3 minutes, not less. It doesn't disprove at all the idea that most of the caffeine is extracted during the first 30-45s. Why would you assume that the speed of extraction during the first 3 minutes was constant? What IS certainly a myth, is that ALL of the caffeine is extracted during the first 30s. All studies show that even after a 3-5 minutes infusion, there is still between 10 to 25% of caffeine left since that's what comes out in a second infusion.
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WizzFeb 4 at 15:59

Where did I assume anything about constant caffeine extraction? Of course it isn't constant. There are MANY other studies out there, which I've looked at. I just cited a couple that were relevant to the question here; the 30-second decaf method isn't actually relevant, but it was brought up in other answers, so I felt I should address it. Other studies have shown 5-30% of caffeine usually is released in the first 30 seconds. In any case, this disproves the "30-second" myth which usually claims 80-85% extraction. And real "decaf tea" by law must usually have 95-98% or more removed.
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AthanasiusFeb 7 at 2:13

You can make a quick cup of "decaf" tea, by letting the bag steep for under a minute, pouring off the water, then pouring more water over it and letting it steep again. The poured off water will contain about 80% of the caffeine. link

So you're probably not going to get a lot more caffeine from a longer steep time.

caffeine is extremely soluble, but dissolving a caffeine crystal in water and extracting caffeine from behind lots of semipermeable membranes (cell walls) are two very different processes, with very different speeds and final outcomes.
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rumtscho♦Sep 5 '14 at 6:30